Detailed explanation of the role of thermistor

Date:Feb 03, 2021

1. PTC thermistor

    PTC (Positive Temperature Coeff1Cient) refers to the thermistor phenomenon or material that has a positive temperature coefficient and a sharp increase in resistance at a certain temperature, which can be used as a constant temperature sensor. The material is a sintered body with BaTiO3 or SrTiO3 or PbTiO3 as the main component, which is doped with a small amount of oxides such as Nb, Ta, Bi, Sb, Y, and La for atomic valence control to make it semiconducting. The semiconducting BaTiO3 and other materials are referred to as semiconducting (bulk) porcelain; at the same time, oxides of Mn, Fe, Cu, Cr and other additives that increase the positive temperature coefficient of resistance are added, and they are formed by general ceramic technology. High temperature sintering makes platinum titanate and its solid solution semiconducting, thereby obtaining positive thermistor materials. Its temperature coefficient and Curie point temperature vary with the composition and sintering conditions (especially the cooling temperature).

    Barium titanate crystal belongs to the perovskite structure, it is a ferroelectric material, and pure barium titanate is an insulating material. Adding trace rare earth elements to the barium titanate material, after proper heat treatment, the resistivity sharply increases by several orders of magnitude near the Curie temperature, resulting in the PTC effect. This effect is related to the ferroelectricity of BaTiO3 crystal and near the Curie temperature The phase change of the material is related. Barium titanate semiconducting porcelain is a polycrystalline material, and there are inter-grain interfaces between the crystal grains. When the semiconducting porcelain reaches a certain temperature or voltage, the crystal grain boundary changes, and the resistance changes sharply.

    The PTC effect of barium titanate semiconducting porcelain originates from grain boundaries (grain boundaries). For conducting electrons, the interface between grains is equivalent to a potential barrier. When the temperature is low, due to the action of the electric field in the barium titanate, the electrons can easily cross the barrier, and the resistance value is small. When the temperature rises to near the Curie point temperature (that is, the critical temperature), the internal electric field is destroyed, and it cannot help the conductive electrons to cross the barrier. This is equivalent to a rise in the potential barrier and a sudden increase in the resistance value, resulting in the PTC effect. The physical models of the PTC effect of barium titanate semiconducting ceramics include the sea view surface barrier model, the barium absence model of Daniels et al., and the superimposed barrier model. They provide reasonable explanations for the PTC effect from different aspects.

    The PTC thermistor appeared in 1950, and then in 1954, a PTC thermistor with barium titanate as the main material appeared. PTC thermistors can be used for temperature measurement and control in industry, as well as for temperature detection and adjustment of certain parts of automobiles. They are also widely used in civil equipment, such as controlling the water temperature of instantaneous boilers, air conditioners and cold storage, Use its own heating for gas analysis and wind speed machine.

    In addition to being used as a heating element, the PTC thermistor can also function as a "switch". It has three functions: a sensitive element, a heater and a switch, and is called a "thermal switch". After the current passes through the element, the temperature rises, that is, the temperature of the heating element rises. When the Curie point temperature is exceeded, the resistance increases, thereby limiting the current increase. Then the decrease of the current causes the temperature of the element to decrease, and the decrease of the resistance value causes the circuit current Increase, the temperature of the element rises and repeats itself, so it has the function of keeping the temperature in a specific range, and it also acts as a switch. Use this resistance to temperature characteristics to make a heating source, as heating element applications such as heaters, electric irons, drying wardrobes, air conditioners, etc., can also play a role in overheating protection for electrical appliances.

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